Modern jets may go above 40 000, but except for some airliners
(767, 747sp, concord) this is still fairly rare. Even for these
aircraft (except concord) the vast majority of cruising would be
done below 40 000.
e.g. 747-400 has an altitude limit of 45 100. Only once in the
last year of flying them have I gone above 40 000 (and then only
for about 2 hours at the end of a particularly long sector).
The troposhere is above 40 000 in the latitudes where most humans
live. At high latitudes the troposhere gets lower. Polar flights
are almost always done abopve the troposhere (and therefore have
fewer weather problems). But very few of all the flights done in
the world are over polar regions.
>From MS Encarta
Troposphere, lowest layer of the earth's atmosphere and the site
of all weather processes. The troposphere extends up to an
altitude of about 11 km (about 7 mi) above the polar zones and to
about 16 km (about 10 mi) above the equatorial regions. The
tropopause forms the boundary between troposphere and
stratosphere.
" Microsoft (R) Encarta. Copyright (c) 1994 Microsoft Corporation.
Copyright (c) 1994 Funk & Wagnall's Corporation.
Thanks Martin Smith